High-strength steels have been actively used to meet requirements for both lightweightedness and high strength, in automobile bodies, with an emphasis on the impact resistance stability regulations and fuel efficiency of automobiles. In accordance with this trend, the application of high-strength steels to automotive exterior panels has also been extended. At present, 340 MPa-grade bake hardened steel is most commonly used in automotive exterior panels, but some 490 MPa-grade steel sheets have been used therein, and this trend is expected to continue to be extended to 590 MPa-grade steel sheets.
As described above, when steel sheets having increased strength are employed as exterior panels, lightweightedness and dent resistance may be improved, while, as the strength increases, formability during processing may decrease. Hence, in order to compensate for insufficient formability while applying high-strength steels to exterior panels, steel sheets having a relatively low level yield ratio (YR=YS/TS) and a relatively high level of ductility have been required by automobile manufacturers.
Furthermore, steel sheets employed as automotive exterior panels are required to have excellent surface quality, but it is difficult to secure plating surface quality due to hardenable elements or oxidizing elements, such as silicon (Si) or manganese (Mn), added to provide high strength.
Moreover, since steel sheets for automobiles are required to have high levels of corrosion resistance, hot-dip galvanized steel sheets having excellent corrosion resistance have been used as steel sheets for automobiles in the related art. Such steel sheets are manufactured by continuous hot-dip galvanizing equipment that performs recrystallization annealing and plating on the same production line, and thus steel sheets having high levels of corrosion resistance may be produced at low cost.
Further, galvannealed steel sheets subjected to a heat treatment after being hot-dip galvanized have been widely used due to having excellent weldability and formability, as well as outstanding corrosion resistance.
Thus, the development of high-tensile cold-rolled steel sheets having excellent formability has been required to improve lightweightedness and processability of automotive exterior panels. In addition, the development of high-tensile hot-dip galvanized steel sheets having excellent corrosion resistance, weldability, and formability has been required.
As a technology in the related art for improving processability of high-tensile steel sheets, Patent Document 1 discloses a steel sheet having a complex-phase structure using martensite as a main component, and a method of manufacturing the high-tensile steel sheet, in which fine copper (Cu) precipitates having a particle diameter of 1 nm to 100 nm are dispersed in a complex-phase structure thereof, to improve processability.
Patent Document 1 requires the addition of Cu in an excessive amount of 2% to 5% to extract fine Cu particles, which may cause red shortness resulting from Cu and an excessive increase in manufacturing costs.
Patent Document 2 discloses a complex-phase steel sheet including ferrite as a main phase, retained austenite as a secondary phase, and bainite and martensite as a low-temperature transformation phase, and a method of improving the ductility and elongation flange properties of the steel sheet.
However, Patent Document 2, it is difficult to secure plating quality due to the addition of large amounts of silicon (Si) and aluminum (Al) to secure a retained austenite phase, and also difficult to secure surface quality during a steel manufacturing process and a steel continuous casting process. Further, transformation induced plasticity allows for a relatively high initial YS value, to increase a yield ratio.
Patent Document 3 discloses a steel sheet including soft ferrite and hard martensite as microstructures, and a manufacturing method thereof for improving an elongation percentage and an r value (a Lankford value) of the steel sheet, as a technology for providing a high-tensile hot-dip galvanized steel sheet having good processability.
However, this technology has difficulties in securing excellent plating quality due to the addition of Si in a large amount, and causes an increase in manufacturing costs because of the addition of titanium (Ti) and molybdenum (Mo) in large amounts.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2005-264176
Patent Document 2: Japanese Laid-Open Patent Publication No. 2004-292891
Patent Document 3: Korean Published Patent Application No. 2002-0073564